The invention relates to curable compositions used for making ene-thiol elastomers and cured ene-thiol elastomers made therefrom.
Electronic circuits must be protected from exposure to harsh corrosive environments to maintain the performance of the electronic device. Many electronic circuits are used in environments where they are exposed to corrosive liquids. For example, adhesives and encapsulants used to assemble ink jet cartridges must protect the flexible circuit that controls the ink jet head from exposure to corrosive inks. These adhesives and encapsulants experience long term exposure to very corrosive inks. If the adhesive or encapsulant degrade or excessively swell, the ink contacts and corrodes the circuit.
Thermosetting resins, frequently epoxy resins, are used to protect circuits from corrosive environments. Epoxy resins have several characteristics that limit their ability to perform well. Traces of chloride ion, which are frequently present in epoxy resins, promote the corrosion of circuits. Epoxy networks are somewhat hydrophilic and swell in aqueous environments because of the secondary alcohols produced in the curing reaction. Epoxy networks are frequently difficult to fully cure in the time/temperature constraints of electronic manufacturing processes. Unreacted epoxy groups are prone to hydrolysis, forming glycols, which further decreases the water resistance of the network. These epoxy characteristics limit their use as adhesives and encapsulants in corrosive environments.
Many combinations of polyfunctional olefins and mercaptans have been used to prepare ene-thiol networks. While many monomers have very attractive process characteristics (low viscosity and rapid UV curing), they do not provide networks having the requisite environmental resistance to withstand highly corrosive aqueous environments. Polyether dimercaptans are frequently used in ene-thiol compositions. These monomers introduce hydrophilic units to the cured network, resulting in excessive swelling in aqueous environments. Multifunctional mercaptoacetates and propionates are other commonly used thiol monomers. In addition to their hydrophilic character, the ester linkage introduces a site for hydrolysis and network degradation.
In one aspect, the invention provides a curable composition for making an ene-thiol elastomer comprising a mixture of a polythiol, or a mixture of polythiols, having at least two thiol groups and free of hydrophilic groups, and an aromatic, heterocyclic, aliphatic, or cycloaliphatic polyene having at least two reactive unsaturated carbon to carbon bonds, wherein the composition in cured form, when immersed in a solution of 96 parts by weight water and 4 parts by weight n-butanol, shows a weight increase of not more than 4 weight percent, preferably, not more than 3 weight percent, and more preferably, not more than 2.5 weight percent in 15 days at a temperature of 22xc2x0 C.
In another aspect, the invention provides a ene-thiol elastomer comprising the reaction product of a composition comprising a mixture of a polythiol having at least two thiol groups and free of hydrophilic groups and an aromatic, heterocyclic, aliphatic, or cycloaliphatic polyene having at least two reactive unsaturated carbon to carbon bonds, wherein the ene-thiol elastomer, when immersed in a solution of 96 parts by weight water and 4 parts by weight butyl alcohol, shows a weight increase of not more than 4 weight percent, preferably, not more than 3 weight percent, and more preferably, not more than 2.5 weight percent in 15 days at a temperature of 22xc2x0 C.
In another aspect, the invention provides a curable composition for making an ene-thiol elastomer comprising a mixture of (a) a thiol terminated oligomer comprising the reaction product of a polythiol having two thiol groups and a first polyene or mixture of polyenes having two reactive unsaturated carbon to carbon bonds, and (b) a second polyene or a mixture of polyenes having at least 5 percent functional equivalents of unsaturated carbon to carbon bonds from polyenes having at least three unsaturated carbon to carbon bonds, wherein the composition in cured form, when immersed in a solution of 96 parts by weight water and 4 parts by weight n-butanol, shows a weight increase of not more than 4 weight percent, preferably, not more than 3 weight percent, and more preferably, not more than 2.5 weight percent in 15 days at a temperature of 22xc2x0 C.
Generally, not more than 50 weight percent, preferably, not more than 30 weight percent, more preferably, not more than 20 weight percent, and even more preferably, none of the polythiol used to make the oligomer has hydrophilic groups. The first and second polyenes or mixtures of polyenes may be the same or different. Preferred first polyenes include divinyl ethers, and cyclic polyenes. Preferred polythiols include dimercaptodiethyl sulfide, 1,6-hexanedithiol, and 1,8-dimercapto-3,6-dithiaoctane.
In another aspect, the invention provides a ene-thiol elastomer comprising the reaction product of a composition comprising the reaction product of (a) a thiol terminated oligomer comprising the reaction product of a polythiol having two thiol groups and a first polyene or mixture of polyenes having two reactive unsaturated carbon to carbon bonds and (b) a second polyene or a mixture of polyenes having at least 5 percent functional equivalents of unsaturated carbon to carbon bonds from polyenes having at least three unsaturated carbon to carbon bonds, wherein the composition in cured form, when immersed in a solution of 96 parts by weight water and 4 parts by weight n-butanol, shows a weight increase of not more than 4 weight percent, preferably, not more than 3 weight percent, and more preferably, not more than 2.5 weight percent in 15 days at a temperature of 22xc2x0 C.
Generally, not more than 50 weight percent, preferably, not more than 30 weight percent, more preferably, not more than 20 weight percent, and even more preferably, none of the polythiol used to make the oligomer has hydrophilic groups. The first and second polyenes or mixtures of polyenes may be the same or different. Preferred first polyenes include divinyl ethers, and cyclic polyenes. Preferred polythiols include dimercaptodiethyl sulfide, 1,6-hexanedithiol, and 1,8-dimercapto-3,6-dithiaoctane.
In another aspect, the invention provides ene-thiol elastomers comprising the reaction product of a composition comprising a mixture of a polythiol having at least two thiol groups and free of hydrophilic groups and an aromatic, heterocyclic, aliphatic, or cycloaliphatic polyene having at least two reactive unsaturated carbon to carbon bonds, wherein the ene-thiol elastomers have a water vapor transmission rate of less than 50, preferably less than 30, more preferably, less than 20 g-mm/m2-day at 40xc2x0 C. according to ASTM D814.
In another aspect, the invention provides ene-thiol elastomers comprising the reaction product of a composition comprising the reaction product of (a) a thiol terminated oligomer comprising the reaction product of a polythiol having two thiol groups and a first polyene or mixture of polyenes having two reactive unsaturated carbon to carbon bonds and (b) a second polyene or a mixture of polyenes having at least 5 percent functional equivalents of unsaturated carbon to carbon bonds from polyenes having at least three unsaturated carbon to carbon bonds, wherein the ene-thiol elastomers have a water vapor transmission rate of less than 50, preferably less than 30, more preferably, less than 20 g-mm/m2-day at 40xc2x0 C. according to ASTM D814.
In another aspect, the invention provides an ene-thiol elastomer comprising the reaction product of (a) an unsaturated carbon to carbon bond terminated oligomer comprising the reaction product of a first polythiol having two thiol groups and a polyene or mixture of polyenes having two reactive unsaturated carbon to carbon bonds; and (b) a second polythiol or mixture of polythiols having at least 5 percent functional thiol equivalents from polythiols having at least three thiol groups, wherein the ene-thiol elastomer shows a weight increase of not more than 4 weight percent in 15 days at a temperature of 22xc2x0 C. when immersed in a solution of 96 parts by weight water and 4 parts by weight n-butanol.
In other aspects, the invention provides a method of making the above ene-thiol elastomers, and an article of manufacture comprising electrical or electronic components encapsulated in a ene-thiol elastomer of the invention.
The compositions of the invention preferably contain a free radical initiator and more preferably, a photoinitiator.
As used herein, the term xe2x80x9cpolythiolsxe2x80x9d refers to simple or complex organic compounds which are substantially free of disulfide linkages and have a multiplicity of pendant or terminally positioned xe2x80x94SH functional groups per molecule.
As used herein, the term xe2x80x9cfree of hydrophilic groupsxe2x80x9d when used to describe polythiols means polythiols devoid of any ether, ester, hydroxyl, carbonyl, carboxylic acid, sulfonic acid linkages or groups within or pendant from the polythiol molecule.
As used herein, the term xe2x80x9cpolyenexe2x80x9d refers to simple or complex species of alkenes having at least two reactive unsaturated carbon to carbon bonds per molecule.
As used herein, the terms xe2x80x9cdi functionalxe2x80x9d, xe2x80x9ctrifunctionalxe2x80x9d, and xe2x80x9ctetrafunctionalxe2x80x9d when used to describe polythiols and polyenes means polythiols having two, three, and four thiol groups and polyenes having two, three, and four reactive unsaturated carbon to carbon bonds.
The compositions of the invention are generally low viscosity liquids that can be uniformly coated onto flexible circuitry and rapidly cured by actinic radiation. The resulting ene-thiol elastomers are tough, flexible, and resist swelling or chemical degradation by water and corrosive components of inks.
One of the unique properties of the ene-thiol elastomers of the invention is the combination of flexibility with resistance to swelling and degradation by water and corrosive environments. Brittle thermoset resins, such as conventional epoxies, may provide reasonable resistance to swelling by corrosive components of inks but are prone to cracking when used on a flexible circuit. The resulting cracks then provide a path for the corrosive liquid to penetrate the coating and corrode the substrate. Low Tg epoxies, acrylates, urethanes or other elastomeric thermosetting resins, which are flexible and resist cracking, are generally prone to degradation by these corrosive liquids. The ene-thiol elastomers of the invention provide the swelling resistance of brittle glassy epoxy resins with elastomeric flexibility.
The polythiols of the invention have at least two thiol groups and are free of hydrophilic groups. Useful polythiols are also substantially free of disulfide linkages that would impart chemical and/or thermal instability to the crosslinked or cured network. The polythiols may be aliphatic or aromatic and may be monomeric or polymeric. Useful polythiols have the formula: 
where m=2-12, n=2-12, q=0-4, where m and n can be the same or different; or the formula Hxe2x80x94Sxe2x80x94Rxe2x80x94Sxe2x80x94H, where R=C5-C8 cycloaliphatic radical.
The use of di-, tri-, and tetra-functional polythiols is also contemplated in the present invention.
Specific examples of useful polythiols include dimercaptodiethyl sulfide; 1,6-hexanedithiol; 1,8-dimercapto-3,6-dithiaoctane; propane-1,2,3-trithiol; 1,2-bis[(2-mercaptoethyl)thio]-3-mercaptopropane; tetrakis(7-mercapto-2,5-dithiaheptyl)methane; and trithiocyanuric acid. The polythiols may be used alone or in combination with one another.
When using polythiols having two thiol groups, useful polyenes of the invention may be characterized by mixtures of those materials having at least 5 percent functional equivalents of unsaturated carbon to carbon bonds contributed by polyenes having at least three unsaturated carbon to carbon bonds. Preferred polyenes are heterocyclic, aliphatic, or cycloaliphatic diene, allyl ether, allyl ester, vinyl ether, styryl, (meth)acryl, allyl or vinyl compounds having at least two or three reactive unsaturated carbon to carbon bonds per molecule. Mixtures of polyenes, each having two and three unsaturated carbon to carbon bonds respectively, are preferred. Specific examples include triallyl-1,3,5-triazine-2,4,6-trione; 2,4,6-triallyloxy-1,3,5-triazine; 1,4-cyclohexanedimethanol divinyl ether; 4-vinyl-1-cyclohexene; 1,5-cyclooctadiene; and diallyl phthalate. Combinations of useful polyenes may also be used in the compositions of the invention. The polythiols and polyenes are present in the compositions and elastomers of the invention in a stoichiometric amount.
The composition of the invention may contain a free radical initiator, preferably a UV active free radical initiator, to cure or crosslink the composition. Useful free radical initiators which are well known in the art and include the class of free radical initiators are commonly referred to as xe2x80x9cphotoinitiators.xe2x80x9d A preferred commercially available free radical initiator, also a photoinitiator, is IRGACURE 651, available from Ciba Specialty Chemicals, Tarrytown, N.J. Alternatively, the compositions of the invention may also contain thermally activated free radical initiators.
The compositions of the invention are generally made by mixing a stoichiometric amount of one or more polythiols and one or more polyenes in an appropriate vessel. In the case of reacting polythiols and polyenes, each having two thiol and unsaturated carbon to carbon bonds respectively, it may be preferable to first form an oligomer using a sub-stoichiometric amount of polyene and then reacting the oligomer with a polyene having at least three unsaturated carbon to carbon bonds to form the crosslinked elastomer. If a photoiniator is used, the components may be mixed in the absence of actinic radiation and then stored in the dark for extended periods of time. If desired, the compositions of the invention may contain conventional inhibitors to prevent spontaneous radical polymerization.
One of the advantages of first preparing an oligomer by reacting a sub-stoichiometric amount of polyene having two unsaturated carbon to carbon bonds with polythiol having two thiol groups is that the oligomer, having an increased molecular weight, may be vacuum devolatilized so to substantially reduce the objectionable odor characteristics of polythiols. The resulting oligomers have very low vapor pressures by virtue of their molecular weight and have little odor, but may contain volatile sulfur containing compounds that cause objectionable odor. The removal of such compounds results in compositions having low odor. Another advantage of first preparing oligomers is that such preparation allows the use of combinations of polyenes having different reactivities. For example, a polyene having two unsaturated carbon to carbon bonds having low reactivity can be used to prepare the oligomer and a mixture of polyenes having two and three unsaturated carbon to carbon bonds having relatively high reactivity can be used to react with the oligomer to form the elastomer.
Alternatively, the ene-thiol elastomers of the invention may be made using a polythiol having either three or four thiol groups per molecule and a polyene oligomer terminated with unsaturated carbon to carbon bonds. Such polyene oligomers can be made from the reaction of a polyene having two unsaturated carbon to carbon bonds and a sub-stoichiometric amount of a polythiol having two thiol groups per molecule. Elastomers can be made by reacting the polyene oligomer with polythiols, wherein at least 5 percent of the functional equivalents of thiol is provided by polythiols having at least three thiol groups per molecule.
The compositions can then be applied to the desired substrate, for example, electrical connectors, or other electrical components and the like, and exposed to electron beam radiation. If the composition contains a photoinitiator, the composition may be exposed to any form of actinic radiation, such as visible light or UV radiation, but is preferably exposed to UVA (320 to 390 nm) or UVV (395 to 445 nm) radiation. Generally, the amount of actinic radiation should be sufficient to form a solid mass that is not sticky to the touch. Generally, the amount of energy required curing the compositions of the invention ranges from about 0.4 to 20.0 J/cm2.
DMDSxe2x80x94Dimercaptodiethyl sulfide (Structure 1), HSC2H4SC2H4SH, CAS No. 3570-55-6, available from Itochu Specialty Chemical Inc.
DMDOxe2x80x941,8-dimercapto-3,6-dioxooctane, HSC2H4OC2H4OC2H4SH, CAS NO. 14970-87-7, available from Itochu Specialty Chemical Inc.
EBMPxe2x80x94Ethylene bis(3-mercaptopropionate), HSC2H4COOC2H4OOCC2H4SH, 7575-23-7, available from Evans Chemetics Division of Hampshire Chemicals.
CAPCURE(copyright) 3-800xe2x80x94Trifunctional mercaptan terminated liquid polymer, available from Henkel Corporation.
HDTxe2x80x941,6-hexanedithiol, HSC6H12SH, CAS No. 1191-43-1, available from Aldrich Chemical Company.
IGRACURE 651xe2x80x942,2-Dimethoxy-2-phenylacetophenone, C6H5COC(OCH3)2C6H5, CAS No. 24650-42-8, available from Ciba Specialty Chemicals.
TAICxe2x80x94Triallyl-s-triazine-2,4,6(1H,3H,5H)-trione, (Structure 2), CAS No. 1025-15-6, available from Aldrich Chemical Company.
TACxe2x80x94Triallyloxy-1,3,5-triazine, (Structure 3), CAS No. 101-37-1, available from Aldrich Chemical Company.
Rapi-Cure CHVExe2x80x941,4-cyclohexanedimethanol divinyl ether (Structure 4), CAS No. 17351-75-6, available from International Specialty Products.
VCHxe2x80x944-vinyl-1-cyclohexene (Structure 5), CAS No. 100-40-3, available from Aldrich Chemical Company.
CODxe2x80x941,5-cyclooctadiene (Structure 6), CAS No. 111-78-4, available from Aldrich Chemical Company.
DAPxe2x80x94diallyl phthalate (Structure 7), CAS No. 131-17-9, available from Aldrich Chemical Company.
PEGDExe2x80x94poly(ethylene glycol) divinyl ether (Structure 8), CAS No. 50856-26-3, available from Aldrich Chemical Company.
AIBNxe2x80x942,2xe2x80x2-azobisisobutronitrile, CAS No. 78-67-1, available from Aldrich Chemical Company. It is used as a thermal free radical initiator.
NPALxe2x80x94tris(N-nitroso-N-phenylhydroxylamine)aluminum salt, CAS No. 15305-07-4, available from First Chemical Corporation. It is used as a radical inhibitor. 
Oligomer Preparation
A variety of oligomers with different backbone structures were synthesized. DMDS was chosen as a monomer to minimize the amount of ether linkages in the backbone and maximize the number of thioether linkages. The oligomers were prepared by the addition of a dimercaptan to a diolefin under free radical conditions. The molecular weight of the oligomer was controlled by reaction stoichiometry. The reactions were carried out either thermally or photochemically. Polymerizations carried out with less reactive olefins such as VCH and COD were more successful using the photochemical method.
Thermal Procedure. A dimercaptan or mixture of dimercaptans was weighed into a flask, AIBN was added, and the flask was heated to 65xc2x0 C. A diolefin was added dropwise to the dimercaptan solution at a rate to maintain the temperature in the flask between 90-100xc2x0 C. After the addition, the oligomer was stirred for 4 hours, and the temperature was maintained at 75-80xc2x0 C. The reaction product was checked by 1H NMR and 13C NMR to determine whether any olefin groups remained. If olefin was still present in the product mixture, additional AIBN was added and the oligomer was stirred at 75-80xc2x0 C. for an additional 5 hours, when the amount of olefin remaining was determined to be less than 4 percent by 1H NMR. The oligomers in Table 1 were prepared using the thermal procedure.
Alternative Thermal Procedure. The reaction was carried out as described above; however, 0.5 percent AIBN was dissolved into the diolefin, and the resulting solution was added to the dimercaptan.
Photochemical Procedure. A dimercaptan, a diolefin, and 0.5 weight percent IRGACURE 651 were weighed into a glass jar. The contents of the jar were shaken and were irradiated for 4 hours with two GTE 15 watt Sylvania 350 nm black light bulbs having a power output of 1 mW/cm2. Additional IRGACURE 651 was added, and the oligomer was heated to 80xc2x0 C. The oligomer was irradiated again for several hours. The reaction was considered complete when greater than 95 percent of the olefin groups had reacted as judged by 1H and 13C NMR. The oligomers in Table 2 were made using the photochemical procedure.
Oligomers 9 and 10 are described in Table 3 and contain a combination of VCH, which contains no oxygen ether linkages, and CHVE, which has reactive vinyl ether groups. DMDS, VCH, and IRGACURE 651 were weighed into a glass jar. The contents were shaken and irradiated for 4 hours with two GTE 15 watt Sylvania 350 nm black light bulbs having a power output of 1 mW/cm2. CHVE and IRGACURE 651 were added to the resulting oligomer, and the solution was irradiated again for 2 hours.
Oligomers 11 and 12 are described in Table 4 and were prepared with a mixture of DMDS and DMDO. The alternative thermal procedure listed above was used, and 0.5 percent AIBN was the catalyst level for each reaction. For Oligomer 11, DMDS and DMDO were used in a 70:30 ratio, and for Oligomer 12 DMDS and DMDO were used in a 50:50 ratio. For both oligomers, there are more sulfur atoms than oxygen atoms in the backbone.
Oligomers 13-18 are described in Table 5 and contain more oxygen atoms than sulfur atoms in the backbone and were prepared for purposes of comparison. The alternative thermal procedure listed above was used, and 0.5 percent AIBN was the catalyst level for each reaction.
Preparation of DMDT. DMDT, 1,8-dimercapto-3,6-dithiaoctane (Structure 9), was prepared from 3,6-dithia-1,8-octadiol using the method of Cooper et al. with the exception that 38 percent HCl was used in place of 48 percent HBr in the synthesis. Wolf, R. E., Jr.; Hartman, J. R.; Storey, J. M. E.; Foxman, B. M.; Cooper, S. R. J. Am. Chem. Soc. 1987, 109, 4328-4335. 